From the beginning of controlled electrical switching, arc pitting and the wear of switch contact points has been a pervasive issue that actually created significant job opportunities over many decades. With the advent of alternating current, point wear was at least somewhat normalized between the two points. And with the development of more powerful electrical machines switching apparatus became remote to the actual user as the economies of smaller wire and small currents were used to remotely switch much larger electrically activated switches.
For many decades, the salvation of remote switching of resistive and especially inductive loads was the mercury-wetted contact relay. The resurfacing of the contacts with every mechanical action provided a switching lifetime rated in the tens of millions of cycles; but recent environmental concerns have eliminated the use of mercury in routine industrial and commercial devices and more so in the household application of remote switching.
Solid state technologies created an opportunity for low power electronics to monitor the alternating current waveforms and attempt to synchronize the switch action of the actual opening and closing with the zero crossing of the waveform. The device speed of emerging solid state devices such as the Silicon-Controlled Rectifier (SCR) or Triac enabled the ambition for a zero-crossing switch. But these devices, at best, have about a 1.5 Volt drop across the device when switched on. This voltage, times the load current, is the energy dissipated as heat. For a 20 Amp load, this is 30 Watts of loss. In today's environmentally sensitive market, 30 Watts per switch is a terrible waste.
Mechanical relays on the other hand do not dissipate significant energy in the “on” state, but unlike SCRs and Triacs, which are easily switched at a zero crossing, the relatively slow mechanical relays form arcs as the contact points open or close while current flows and the resulting heat melts or pits a little of the contact points at every switch cycle. The contact points have to be specifically designed to be “self cleaning,” in which case they are designed to wear out but maintain a low “on” resistance; or they are dramatically over-built in anticipation of wear. In either case, mechanical relays have a significant wear rate.
As of 2017 opportunities to extend useful functionality through the “Internet of Things” brings heightened sensitivity to the efficiencies of remote switching. Additionally over the last 20 years an increased awareness of the capability to provide life safety capabilities in circuit extension and switching devices has brought forth first Ground Fault Interrupter National Electric Code requirements for such devices. And in the last few years the requirement for Arc Fault Interrupter protection for property has become a reality.
This invention is a logical extension of the history of robust switching, the technology of embedded control, the emerging needs of an Internet-based remote switching activity, and the evolving concern to provide circuit safety as well as safety for life and property in the immediate environment of the switching activity.
Early inventions targeting more robust switching were focused on the monitoring and detection of the alternating current zero crossing as a switch point opportunity. Michael Sidman's U.S. Pat. No. 4,153,870 granted on May 8, 1979, revealed a method and apparatus to identify the zero crossing in a power circuit switched by a Triac or back-to-back SCRs.
Of significant achievement was the disclosure by Rockwell International of a monolithic solid state power controller featuring zero crossing switching and programmable or configurable current limiting in U.S. Pat. No. 4,174,496 by inventors William McFall et al, granted on Nov. 13, 1979.
A disclosure using lower-cost components for a zero-crossing switch was granted to inventor Raymond Robertson on Aug. 14, 1984, U.S. Pat. No. 4,466,038. In this invention a single unidirectional SCR is used to isolate and control circuit switching for half cycles, allowing for a near zero voltage switching opportunity to energize and close a mechanical switch.
In U.S. Pat. No. 4,767,944 granted on Aug. 30, 1988, Hiroto Takeuchi and Masahiro Hishimura confirmed that analysis had shown switch contact points to be most affected by the operation of opening under a load, as the AC cycle was more than π/8 away from a zero-crossing point. Their method supported a novel apparatus which detects and measures the exact phase angle of the alternating current and drives a mechanical relay to only open when within π/8 of a zero crossing in the alternating current phase angle. Prior to this invention, the option of using a solid state switch in AC circuits was the most likely way to extend contact point life. But Takeuchi's invention significantly extended point life and did not generate the thermal loss and heat build up as did solid state switching.
Another unique extension solution to the switch point wear issue was disclosed by Hiroyuki Nishi et al in their Oct. 1, 1991, U.S. Pat. No. 5,053,907 in which three sets of contact points are synchronized to the alternating current waveforms of three phases and operate in conjunction with three triacs for the control of three-phase motors and other equipment. This device also includes circuitry to suppress leakage current of the electronics.
The true “hybrid” relay was first disclosed by Andrew Kadah in his Dec. 16, 1997, U.S. Pat. No. 5,699,218 which used a TRIAC as the solid state switching device and a mechanical relay to carry the load current. In this method the leakage current is minimized by capacitor coupling the gate drive to the TRIAC.
John Dougherty in his U.S. Pat. No. 6,046,899 of Apr. 4, 2000, presents a method of simultaneous operation of both a solid state switch and a mechanical switch where actuation of the two devices is achieved by a common power feed. This method relies on the inherent speed of the solid state device to close or open the circuit before the mechanical components have physically connected or broken the circuit and had opportunity to arc the contact points.
Gerard Blain and Luc Raffestin, in their U.S. Pat. No. 6,347,024 B1 of Feb. 12, 2002, revealed a method of simultaneously energizing the solid state switch and the mechanical relay on the same alternating current cycle. This method incorporates a programmable device that can be configured to sequence the solid state switch and the mechanical relay in response to the relative speed of a variety of solid state switches by adjusting the timing between control signals for each particular type of switch.
Hervé Carton and Denis Flandlin, in their U.S. Pat. No. 6,643,112 B1 of Nov. 4, 2003, reveal a novel implementation of a single transistor used to eliminate the point arc of a mechanical relay. In this implementation the transistor is used in parallel with a mechanical relay to eliminate point arcing in DC circuits. And to detect the alternating current direction and only permits a mechanical switch closing or opening when the alternating current is flowing such that the transistor is forward biased and configured to initiate or terminate current flow at the next zero crossing.
Sergio Orozco in his U.S. Pat. No. 8,089,735 B2 of Jan. 3, 2012 further evolved both the method and the apparatus by integrating the temperature measurement of the solid state switch into the control process and protectively disabling the switch as the temperature rises before the solid state device is damaged by excessive heat.
Common to the significant evolutionary steps of robust switching is a relatively narrow focus towards maximizing the useful life of the switch and minimizing the operational thermal losses of the switching activity. The early Rockwell device (U.S. Pat. No. 4,174,496) stands out uniquely as an attempt to combine circuit protection with zero-crossing switching by implementing a configurable current limiting activity as an integrated functionality.
Recent technology developments have enabled the monitoring of branch circuits to not only add protection for equipment but also provide precise monitoring and control to protect human and animal life from the dangers of electric shock or electrically ignited fire. These concerns are now addressed in Ground Fault Circuit Interrupters and ARC Fault Circuit Interrupters.
The invention disclosed herein includes not only the features of a robust switch and a configurable means for circuit protection, but also includes the features of life safety (Ground Fault Interruption) and property safety (Arc Fault Interruption). This collection of features fills an important need when confronted with the opportunities of the “Internet of Things” and the intended applications of remote switching where, because of their nature, the remote components of Internet implementations imply that the only observational opportunities will be built-in to the controller.
Although the telepresence sensors are not required for a full implementation, this disclosure will illustrate that the basic apparatus configuration is constructed to provide extended environmental sensors as support to the effective management of the remote switch. Support sensing such as temperature, audio and ambient light measurement will greatly enhance the effective remote management of “Internet of Things.”
This invention comprises both an apparatus which is an exceptionally robust, efficient, safe and reliable remote switching device for controlling AC-powered circuits and a method to operate said device. This device nearly eliminates the thermal dissipation common to all SCR (Silicon Controlled Rectifiers) and Triacs (Back-to-Back SCRs for full wave switching) and also nearly eliminates the wear caused by arcing of the contact points in a relay switching device. This invention includes an embedded microcomputer and sensing apparatus and a method which will schedule and control the interleaved switching action of an SCR or Triac electronic switching device and actuation of a mechanical relay creating a “Hybrid Relay,” while monitoring the load and return currents for life safety and circuit current waveforms for distortion caused by arcing: thus a “Robust Safe Switch.”
Still this “Robust Safe Switch,” because of its intended support for the “Internet of Things” and such end effectors as motors, solenoids and other inductive loads, has the configurable ability to accommodate time-limited start-up current ramps that will exceed the normal operating current limits, while maintaining vigilant management of life safety and property safety issues.